The present invention relates to a material used to facilitate the delivery and controlled evaporation of a liquid cryogen. Shaped articles of the present invention are capable of containing and delivering a cryogenic fluid. These articles have a porous structure that restricts the passage of cryogenic fluid in the liquid phase while permitting the passage of cryogenic fluid in the gaseous phase. Such fluids may include nitrogen, helium, hydrogen, argon, neon and air as well as liquefied petroleum gas or low temperature liquids.
By xe2x80x9crestrictxe2x80x9d or xe2x80x9crestrictionxe2x80x9d in this context is meant that while gas can exit a material of the present invention through its exterior surface, liquid will enter into the thickness of the material but will not pass as a liquid through its exterior surface under specific operating conditions (e.g., temperature, humidity, pressure, etc.).
By xe2x80x9clow temperaturexe2x80x9d in this context is meant a temperature substantially below 0xc2x0 C. Typically liquid nitrogen, for example, is liquid at temperature of approximately 77 Kelvin (xe2x88x92196xc2x0 C.) at an atmospheric pressure of one atmosphere.
Two primary technologies are used for the transport or storage of cold liquids or liquids with a low heat of vaporisation, namely, those utilising vacuum insulation and those that operate by dry gas retention. Unlike articles of the present invention, neither of these technologies controls the release of gaseous cryogenic fluid through the exterior surface of the container or conduit.
U.S. Pat. No. 5,511,542 (Westinghouse Electric Corporation) discloses a garment incorporating a conduit constituted by, for example, a Dacron(copyright) tube surrounded by a sheath of non-w oven cotton. The conduit is stated to be impermeable to liquids but permeable to gases. A conduit of this nature is unlike conduits in accordance with embodiments of the present invention. Cryogenic liquids enter the structure of conduits of the present invention and at high enough pressures liquid cryogens leak through the conduit walls. At pressures lower than those that cause liquid leakage through the walls, cryogenic fluid in the gaseous phase exits the exterior surface of the conduit as evidenced by a plume of water condensate.
Cooling garments, such as the Cooling Suit supplied by Aerospace Design and Development, Inc. Niwot, Colo., as part of the SCAMP(copyright) (supercritical air mobility pack) model number 547-000-06 require the use of a coolant that primarily remains in a liquid phase. These garments require a fluid control and heat exchange system, which is heavy. In addition to the extra weight to be carried, such a system has the significant disadvantages of high purchase and service costs. Cooling garments of the present invention possess advantages over cooling garments in the prior art. These advantages include lower weight, lower volumes of liquid coolant used, simpler system control requirements and no need for pumps or fans and their associated power and control requirements.
Various polymers are known to be useful under low temperature conditions such as 77 Kelvin. For example, porous polytetrafluoroethylene (PTFE) is known to retain strength and flexibility at low temperatures, particularly in the form of porous expanded PTFE (ePTFE) constituted by nodes interconnected by fibrils as described in U.S. Pat. No. 3,953,566 to Gore. Such ePTFE, however, is not normally suitable for the transport or storage of cryogenic liquids because of its porosity, which allows cryogenic liquids to have ready passage into and through the ePTFE material.
Temperature gradients affecting materials used in systems such as those involving cryogens are such that thermal expansion and contraction effects cause early mechanical failure in components. Preferred embodiments of this invention, in addition to possessing certain permeation characteristics, relate to materials that retain flexibility and strength at low temperatures, typically 77 Kelvin.
According to one aspect of the present invention, there is provided a material for the transport of a cryogenic fluid, said material having a porous structure which allows a liquid cryogenic fluid to enter through a first surface of the material into the thickness of the material but restricts leakage of liquid cryogenic fluid through the exterior, or second, surface. The first and second surfaces are separated by the thickness. The restriction may occur within the thickness of the material and/or at the exterior surface at the first and/or or interior surface. Furthermore, the material preferably also controls passage of the cryogenic fluid in gaseous phase through the exterior surface of the material.
In its preferred form, the invention provides a liquid permeation restriction material that preferably is lightweight and flexible at low temperatures. It allows evaporative cooling using liquid cryogenic fluids, which affords more efficient cooling than by simply transporting and delivering a gaseous cryogenic fluid. Articles formed of material of embodiments of the present invention afford the ability to transport a liquid cryogen to a specific site, then cool that site by means of conduction from the cold material and convection of a cold gas. The heat loss is greatly enhanced by the phase change of the evaporating liquid.
According to a further aspect of the present invention there is provided a garment incorporating a conduit of a material with permeation qualities as set out in the preceding paragraphs.
Preferably, the material of the present invention is in the form of a tube.
Preferably also, a plurality of layers of material are superimposed on each other to provide a multi-layered composite material possessing a spiral-shaped cross-section, formed from one or more sheets of film. Furthermore, a tube possessing a spiral-shaped cross-section may be comprised of more than one type of film.
The porous material of the invention results in a product which preferably has a high restriction to the through flow of liquid through the wall of the material whilst having a low content of solid material. This preferred material provides improved mechanical and permeation characteristics particularly when used in a multi-layered construction. A multi-layered construction may result in an article that exhibits low bending stresses, thereby increasing its fatigue life. The summation of several layers of material may also increase the pressure required to force liquid cryogen through to the exterior surface.
The material of the present invention may be utilised to restrict liquid cryogen permeation through the material to a rate that will facilitate heat loss through liquid to vapour phase change within the material and at the external surface of the material.
Cryogenic fluid permeation articles made from material of the present invention enable the passage of the gaseous phase of cryogenic fluids across the thickness direction of the article, while inhibiting the passage of the liquid phase of the fluids across the thickness direction. In these articles, the mass flow rate of the liquid phase of a cryogenic fluid flowing through the wall in the thickness direction is less than or equal to the mass evaporation rate of the liquid at the outer wall surface. The material may be modified to alter the restriction of liquid phase cryogenic fluid passage and the controlled release of gaseous phase cryogenic fluid through the exterior of the material. A preferred article in the form of a cryogenic fluid permeation tube has a liquid nitrogen leak pressure (LNLP) (based on the test described below) of at least 0.3 psi (0.002 MPa). Such a tube performs satisfactorily in a cryogenic cooling garment, tested in a manner described below. The tube does not leak liquid nitrogen during the 15 minute test duration. Tubes preferred for use in a cryogenic cooling garment possess a LNLP of at least 0.3 psi (0.002 MPa) and do not fracture during flexure at cryogenic temperatures. Tubes having higher values for LNLP and that do not fracture at these temperatures are more preferred for use in this application; a more preferable tube for use in a cooling garment possesses a liquid nitrogen leak pressure (LNLP) such as 0.45 psi (0.003 MPa).
Any suitable porous material may be used, including polymers, metals, ceramics and mixtures or composites thereof. Fluoropolymer is considered suitable and porous expanded PTFE (ePTFE) is a particularly preferred material because of its flexibility at cryogenic temperatures and the ability to fabricate a tube and other forms from ePTFE with a desired permeability. Although ePTFE is not brittle at very low temperatures, care must be taken in the construction of tubes, and other forms, to ensure that the structure or density of the final tube does not lead to fracture at these temperatures. Non-porous tubes not only typically possess extremely poor permeation properties, they also tend to be unacceptably stiff and prone to fracture, especially at cryogenic temperatures. Low porosity tubes also appear prone to fracture at cryogenic temperatures.
PTFE-based articles of embodiments of the present invention are also preferred because of the low thermal conductivity of PTFE, which is about 0.232 Watts/m.K. Porous articles of PFTE exhibit even lower thermal conductivity. The use of low thermal conductivity materials may result in safer articles with regard to issues such as potential for cold burns. Cryogenic fluid systems will benefit from lower thermal energy ingress and resulting reduction in gas generation within the fluid transport lines. PTFE additionally has a low heat capacity, 1047 kJ/kg K.
The choice of precursor ePTFE film material is a function of the desired number of layers in the final tube, tube wall thickness, air permeability and pore size of the final tube. Pore size may be assessed by isopropanol bubble points (IBP) measurements. Films possessing high IBP values appear to produce final tubes with higher values for LNLP. The use of smaller pore size films appears to increase the LNLP of the final tube. Increased number of layers and increased film thickness may also increase the LNLP of the final tube. The number of layers is preferably between 8 and 48, more preferably between 12 and 24. The LNLP is preferably between 0.003 and 0.075 MPa, more preferably between 0.04 and 0.06 MPa. An ePTFE base tube may also be part of the construction, but the inclusion of a base tube appears not to be critically important. A suitable tube has been constructed using a porous ePTFE film possessing a thickness of 0.0035 inch (0.09 mm), a 39.5 Gurley number and 48.5 psi (0.334 MPa) IBP.
Externally applied reinforcement in the form of rings or helically applied beading or filament or other configurations or materials may be incorporated into the tube construction in order to provide kink and/or compression resistance to the article.
An article in accordance with an aspect of the invention in the form of a membrane suitable for allowing the passage of the gaseous phase of a cryogenic fluid while restricting the passage of the liquid phase of the same cryogen may be produced by a similar process. Multiple layers of film may be wrapped onto a large diameter mandrel, the ends restrained and the assembly placed in an oven in order to bond the layers together using the films and process temperatures described in the examples below. The large diameter tube thus produced may be slit longitudinally to provide a flat membrane. Other techniques may be employed to bond film layers to produce a membrane. The resultant membrane may be used to create more complex shapes, such as pouches, flat constructions with predefined conduits therein or as a liner for storage tanks.
Other articles made from material of the present invention may be useful for cooling warm objects, such as electronic devices, engines, motors, heated elements, and so forth.